Activity: Introduction to Instrumentation: Building a Motor/Propeller Testing Device (a force gauge)

SUMMARY:

Students will learn the engineering design process through the creation of their own force gauges. These force gauges will be used to determine the best propeller/motor combination, per the request of a mock engineering company. They will also learn about measuring applied forces and techniques of technical writing.

LEVEL OF DIFFICULTY [1 = Least Difficult : 5 = Most Difficult]

4- Difficult

TIME REQUIRED

10 - 40 min class periods (varies depending on class)

COST

\$150.00 per class (12-15 students). Cost does not include batteries, alligator clips, or standard masses. Using power sources instead of batteries will significantly reduce cost. (All materials can be reused for Fan Car Activity & future classes)

STANDARDS:

1.1 Identify and explain the steps of the engineering design process, i.e., identify the problem, research the problem, develop possible solutions, select the best possible solution(s), construct a prototype, test and evaluate, communicate the solution(s), and redesign.
1.2 Demonstrate knowledge of pictorial and multi-view drawings (e.g. orthographic projection, isometric, oblique, perspective) using proper techniques.
1.3 Demonstrate the use of drafting techniques with paper and pencil or computer-aided design (CAD) systems when available.
2.1 Distinguish among tension, compression, shear, and torsion and explain how they relate to the selection of materials in structures.

WHAT WILL THE STUDENTS LEARN?
The Engineering Design Process
Group communication skills
The differences between tension, compression, shear and torsion
Application of the laws of physics pertaining to Forces (Hooke's Law)

BACKGROUND INFORMATION:

This activity should coincide with a lesson on applied forces and work including spring constants and Hooke's Law. (Fan Car would be a Good follow up Activity)

There are two ways to transfer energy into or out of a mechanical system, work and heat. Work transfers energy using force and motion, while heat is energy transfer due to a difference in temperature. This activity concentrates on energy transfer due to work, i.e. force and motion.

Science and engineering are both quantitative. We want to know not just that work is done, but how much work is done; we need hard and fast numbers with their proper units. We need to measure things, to record and analyze the data, using data to drive our design changes. Measuring can be easy if you have the right instrument. Take length , for instance; the right instrument, a ruler, is simple to use, easy to obtain and measuring length is a snap.* Applied tension and compression forces can be measured as well using an instrument called a force gauge.

A force gauge can use different methods to measure the force produced by a certain action. In this case, the force will be generated by a motor and propeller apparatus. The objective is to have students design a device that will measure the amount of push or pull that a fan blade apparatus will produce.
*Of course, the difficulty in taking any measurement relates directly to the accuracy and precision necessary. With regard to precision, measuring lengths to the nearest micrometer is obviously much more difficult than measuring to the nearest centimeter.

MATERIALS:
Per group:
2-1/2" Extension Springs, 0.04 lbs/in.,0.04 lbs. I.T., McMaster-Carr prt# 9654K49
2-1/2" Extension Springs, 0.20 lbs/in.,0.21 lbs. I.T., McMaster-Carr prt #9654K63
1-7/8" Extension Springs, 0.57 lbs/in.,0.30 lbs. I.T., McMaster-Carr prt #9654K45
1-1/2" Extension Springs, 0.09 lbs/in.,0.04 lbs. I.T., McMaster-Carr prt #9654K45
12" Ruler
5" Nose Cone Propeller (3 blade), Kelvin part #850891
5" Propeller (2 blade), Kelvin prt # 850632
7" Propeller (2 blade), Kelvin prt #850890
Hi-Speed DC Motor, Kelvin prt #850647
Hi-Performance, Hi-Torque Motor, Kelvin prt #850887
2- 8 1/2 x 11" pieces of paper
Assorted Balsa and Pine Hobby size wood pieces
Alligator clip wires
9V Batteries, power sources
Duct Tape
Hot glue gun, glue sticks
String, twine
Standard masses, 1g. - 200g., mulitple sets

Per Class:
Sample Force Gauge, to be shown after the activity

PREPARATION:
Collect materials; create your own Force Testing Device

DIRECTIONS:

(See Problem Statement Worksheet)
The students should work in groups of two or three.
Following lectures on force, work, and energy, each group should design and build a force gauge device that will help them to choose which combination of motor and propeller to use in a new fan car prototype. Each group should be careful to follow all the steps of the Engineering Design Process including: identify the problem, research the problem, develop possible solutions, select the best possible solution, construct a prototype, test and evaluate, communicate the solution (See Force measuring device and fan testing lab report guidelines), and redesign.

There are two viable options for testing the forces produced by the motor and propeller combinations. One device involves suspending the motor and propeller from a spring and measuring the amount of displacement in the spring as the amount of force being produced. To do this, the spring must be calibrated using known masses. For example, hang a 20g. mass from the spring and record how far the spring stretches. You now know that that distance corresponds to a 0.196 N force. Some considerations in this design are ensuring the torque produced by motor does not "wind" the spring. In addition, balancing the fan blades is important to reduce the amount of vibration in the system, making data collection more difficult.

The second viable method for testing the forces is to set up a balance. This device is a simple lever with a fulcrum in which the motor and propeller push at one end and known masses balance out the force at the other end. A lecture on moments and torque should be included for this design to help the students understand the relationship between distance and forces in this case. A balance device has proven to produce quite accurate results thus far.
The teacher should try to construct both of these designs before introducing the project to the class; this will create a familiarity with the designs and help with trouble shooting in the future. Designs should stay away from measuring something in the airstream created by the propeller because air currents will corrupt the data produced.

Other options and equipment should be made available for the students. The motors should be powered using either a 9V battery or a power source set to 9V of direct current; using power sources will greatly reduce cost. See attached worksheet titled Internal Instrumentation Solutions Group Memo for some other project rules and guidelines. Each group of students were hired as instrumentation specialists to design and construct the instrumentation apparatus and perform testing on the given motors and propellers.

The collected data should be the forces produced from all possible motor/propeller combinations. The students should have sufficient data to conclusively decide which motor/propeller combination produces the most force.

INVESTIGATING QUESTIONS:
Which combination of motor and propeller should the Volpe Center use to power their prototype car?
What is the best way to figure this out?
What do you need to measure and how will you measure it?
REFERENCES:
Developed by: Alfred Fordiani and Douglas Prime

WORKSHEETS: